Method and device for tracking eye movements

ABSTRACT

The present invention relates to a method and a device for tracking eye movements. The aim of the invention is to provide a method and device for tracking eye movements that ensure a reliable tracking of eye movements with the simplest possible operation. To this end, the irradiation ensues via the pupil.

The present invention relates to a method and a device for tracking eyemovements.

[0001] Refractive laser surgery of the cornea, in particular withablative excimer lasers (ArF—wavelength 193 nm) as OP laser, has becomean established method of treating refractive sight defects. Thetreatment is carried out with so-called spot scanning systems on thepatient's freely-moving eye. These systems also allow patient-specificcorrections, so-called customized ablations.

[0002] A patient's eye movement makes it necessary to guide the beam ofthe OP laser in line with the movement. To this end the patient's eyemovement and thus the current eye position is recorded with a so-calledeye-tracking system and used as adjustment value to steer the OP laser.Customary eye-tracking systems work on the basis of image-processingsystems in which these can for example measure the pupil of the eyeusing a digital camera and determine the eye position withimage-processing algorithms by comparing image sequences. As eyemovements take place very rapidly and as even the smallest angle errorshave to be recognized, high-speed video cameras are normally used forthis purpose.

[0003] The pupil is preferably used as tracking object for thehigh-speed video camera, as an automatic video-supported pupil centeringcan thereby be carried out at the start and the same object can also betracked during the treatment. This procedure is problem-free on anuntreated eye. However, already during preparatory operations such asthe removal of the epithelium during PRK (photorefractive keratectomy)or lifting up the flap during LASIK (laser-assisted in-situkeratomileusis), the contrast ratios are changed. During the lasertreatment this problem is further aggravated, as a strongly dispersivesurface beyond the diameter of the pupil is created.

[0004] To avoid this problem different optimization approaches have beenworked out. There is a switch to other tracking objects on the eye suchas e.g. limbus or iris after initial pupil centering or additionaltracking rings are placed on the eye in order to ensure a stablefunctioning of the eye tracking during the treatment. However, thisentails unwelcome additional treatment costs.

[0005] In another optimization approach the eye is illuminated from theside with infrared LEDs. A lower failure rate of the system is therebyalso achieved during the laser treatment.

[0006] However, the illumination necessitates additional work for thedoctor and a limitation of working freedom in the OP field.

[0007] The object of the present invention is therefore to provide amethod and a device for tracking eye movements which ensure a reliabletracking of the eye movement while being as simple as possible to use.

[0008] This object is achieved by a method according to patent claim 1and a device according to patent claim 9.

[0009] The method according to the invention provides that theirradiation takes place through the pupil or the pupil forms aquasi-monochromatic IR source through the reflected light.

[0010] In a further embodiment of the method it is provided that theexcitation of the inside of the eye for illumination takes place througha light source arranged coaxial to the optical axis of an OP laser. Thismethod has the advantage that the operating doctor need only handle asingle radiation source.

[0011] In a further embodiment of the method it is provided that thelight source is an IR laser diode. In this way an adequate lightintensity can be achieved without disruptive light reflexes for examplefor the operating doctor.

[0012] In a further embodiment of the method it is provided that the IRlaser diode operates pulsewise. This allows a high instant-radiationoutput with a correspondingly high brightness of the reflected lightwith a comparatively low average irradiation output.

[0013] In a further embodiment of the method it is provided that thepulses of the IR laser diode are synchronized with the laser-firingfrequency of the OP laser such that the IR laser diode illuminates onlyin laser firing pauses of the OP laser. This measure reduces possibleinterferences with or impairments of the camera lens by the OP laserused.

[0014] In a further embodiment of the method it is provided that thepulses of the IR laser diode are synchronized with the image frequencyof a camera which absorbs the light emitted by the eye. The cameratherefore only records images which are generated by the IR laser diodeduring irradiation of the eye. This measure further reduces inaccuraciescaused by images produced by the OP laser.

[0015] In a further embodiment of the method it is provided that theradiation of the laser diode has a focal point on the surface of thecornea of the eye. In this way the best use is made of the opticalproperties of the eye itself.

[0016] In a further embodiment of the method it is provided that theradiation of the laser diode has a spot which is broadened as muchpossible on the retina of the eye. This measure optimizes the intensityof the back-scattered light.

[0017] The object mentioned at the start is also achieved by a devicefor tracking eye movements, in particular during refractive lasersurgery, comprising a camera, an evaluation unit and also a lightsource, which can operate according to a method according to one ofclaims 1 to 9. In a preferred version it is provided that the radiationof the light source and the light reflected from the eye run essentiallycoaxial. In this way all the elements to be used during the refractivelaser surgery of the eye can be grouped in a single housing and thusonly one device is to be operated by the doctor.

[0018] In a further embodiment of the device it is provided that theradiation of the light source and the light reflected from the eye runessentially coaxial. This measure makes it possible to further combinethe components in only one housing unit.

[0019] In a further embodiment of the device it is provided that thereflected light is guided by a scanner mirror onto the camera arrangednon-coaxial relative to the radiation of the illuminating unit. Usingthe scanner mirror, the two beam paths can thus be split well away fromthe eye.

[0020] The scanner mirror is preferably designed such that it admitsmost of the light striking it from the light source and the eye is thusirradiated and the light reflected from the eye is reflected. Mirrorscan also be optimized with dielectric coating only for transmission orreflection. Because of the lesser light intensity reflected by the eyeand the possibility of choosing virtually any light source output, thescanner mirror is optimized for the reflection in the presentapplication.

[0021] In a further embodiment of the device it is provided that afilter is arranged in the beam path of the reflected light whichessentially passes light of the wavelength emitted from the lightsource. Perturbing radiation which strikes the eye from the surroundingarea and is reflected by the former can thus be filtered out. Theintensity of the light irradiated from the light source onto the eyecannot be increased at will out of consideration for the load-bearingcapacity of the retina of the eye. An improvement of the perturbingradiation distance is therefore effected here by filtering out theinterference spectrum. This measure increases the contrast ratio betweenpupil and surrounding area.

[0022] A dielectric filter element is preferably used with which thesurrounding intensity is strongly suppressed because of themonochromatism of the laser radiation and thus a good contrast ratio canbe produced.

[0023] In a further preferred embodiment of the present invention apoint of fixation on the cornea is chosen to determine the eye position.The purpose of such a point of fixation is to enable the eye movement tobe tracked and to ascertain an “offset” for the current coordinates ofthe eye with reference to this point. The firing parameters andcoordinates can then for example be ascertained by way of deviation fromthis point.

[0024] This point of fixation is particularly preferably the geometriccentre of the optical image of the pupil. This geometric centre orcentre of gravity of the pupil is then taken as point of fixation. Sincethe diameter of the pupil changes, this centre shifts over time. Thiscentre or point of fixation is preferably regularly redetermined. Thusthe point of fixation is kept on the cornea. The “offset” which can forexample be transmitted to a scanner mirror is then determined from theallocation of this point of fixation.

[0025] In a further preferred embodiment of the present invention thelateral displacement of the eye or the rolling movement of the eye isrecorded by evaluating the image of the pupil. Spherical objects, suchas for example the eye, can thereby be treated with lasers, accountbeing able to be taken of whether the ablation spot of the laser isdirected towards the centre of the ball or the eye, or the laser firing,in its notional extension, passes well away from the centre of the eyeor the ball. The efficiency of the ablation spot of the laser depends onthis. If this strikes the ball surface peripherally on the edge, i.e.the notional extended laser firing passes by the centre of the eye, thena higher energy must be expended for the same ablation effect, as thelaser spot then strikes the ball or surface of the eye as an ellipticalprojection of the circular spot. If the firing is targeted precisely onthe centre of the ball or the eye, the efficiency of the laser spot isthen at its highest in the ablation. By taking into account the rollingmovement of the eye as well as the translatory or lateral displacement,these energy effects can be equalized and precisely taken into accountin each case in consideration of the optimum efficiency using theposition of the eye and of the ablation spot to be applied. Thus an evenmore accurate treatment of the surface is possible. The unrolling orrolling of the eye can be ascertained by image processing, in particularvia the evaluation of the recorded structure of the iris. The anteriorchamber depth of the eye is preferably also taken into account. Theangle of rotation of the eye can then be ascertained and this effectthus also taken into account with regard to the energy input on the ballsurface outside the centre.

[0026] In a further preferred method of the present invention acompensation factor is calculated from an average eye length and anaverage anterior chamber depth, which corresponds to the parallaxbetween the cornea front surface and entrance pupil. The deviationswhich can result from the rolling movement of the eye can thereby becompensated. An average value for the biometric parameters such as eyelength and anterior chamber depth is preferably used here.Time-consuming pre-measurements can therefore be avoided, which leads toa swifter implementation of the method.

[0027] The compensation factor is preferably transmitted onto scannermirrors of a working laser. The values obtained to compensate for theparallax can be directly transmitted onto the scanner mirrors whichdeflect a working laser beam and thus ensure that the parallax can bedirectly compensated for on the ball surface to be treated.

[0028] The compensation factor is preferably linear. For small angles inthis geometrically measured parallax for example the approximation istrue that the sine can be replaced by the argument. In addition, furtherapproximation possibilities exist which allow a simple determination ofthis compensation factor. This linear (approximated) compensation factorcan be easily and quickly measured and thus allows a swiftimplementation of the method according to the invention.

[0029] The compensation factor particularly preferably obeys anon-linear function. This non-linear function for establishing thecompensation factor is the geometrically correct function, while thelinear relationship represents an approximation. It is precisely forlarger angles in the parallax that the approximations can lead toundesired deviations or errors. Greater deviations can thus be bettercompensated with the preferred method.

[0030] The compensation factor is preferably determined by theindividual measurement of biometric parameters. Such biometricparameters are in particular the eye length and/or the anterior chamberdepth. The eye length is particularly preferably individually determinedas it is easy to measure. The anterior chamber depth is (also) quiteparticularly preferably individually determined as a particularlycorrect compensation can be individually carried out using same.

[0031] Further advantageous designs of the invention will be explainedin the following using the drawings. There are shown in

[0032]FIG. 1a a schematic diagram of the structure of the device;

[0033]FIG. 1b a schematic diagram of the structure of a furtherembodiment of the device according to the invention;

[0034]FIG. 2 a schematic diagram of the beam path of the device.

[0035] A device for tracking the eye movement 1 includes a light source2, and a telescope 3. The light emitted from the light source 2 isguided via the telescope 3, which can be equipped with further opticalapparatuses, for example such as lenses or similar, to an eye 4 of aperson to be treated, not shown. The light is concentrated in a focalpoint 5. The focal point 5 lies on the cornea surface 6 of the iris 7 ofthe eye 4. After passing through the cornea surface 6 the light beam ofthe light source 2 broadens and strikes the retina 8 of the eye 4 as abroadened spot. The light source 2 is an infrared laser diode in thepresent embodiment. Its output is chosen such that the permissibleirradiation intensity for the retina 8 is observed.

[0036] The light striking the retina 8 is diffusely reflected by same,as indicated by the arrow 9 in FIG. 1. The light 9 reflected from theretina 8 leads to a homogenous illumination of the pupil 10.

[0037] After the reflected light 9 has passed through the pupil 10, itstrikes a scanner mirror 11. This is movable about at least two axes,for example about the lateral axis 12 projecting from the plane of thedrawing and the vertical axis given the reference number 13 in FIG. 1.Through movement about these two axes the scanner mirror 11 can projectdifferent areas of the retina 8 of the eye 4 onto a camera 14. Thecamera 14 is connected to an evaluation unit 15 which controls thescanner mirror 11 on the one hand and an OP laser 16, not shown in moredetail, on the other. The optical axis of the OP laser 16 is arrangedcoaxial to the optical axis of the device for eye tracking 1.

[0038] The scanner mirror 11 is designed such that it passes only asmall part of the light, for example roughly 2%, and reflects thegreater part of the light, for example roughly 98%. Alternatively, thescanner mirror 11 can be provided with a small hole, or the camera 14can observe the eye 4 in the non-coaxial beam path.

[0039] The infrared laser diode of the light source 2 is operatedpulsewise. The light pulses are synchronized, using the evaluation unit15, with the laser firing frequency of the OP laser 16. The resultantpulsewise illumination by the IR laser diode minimizes the loading ofthe retina 8 and increases the contrast between illuminated andnon-illuminated areas.

[0040] The infrared laser diode of the light source 2 emits amonochromatic light. The light reflected from the eye has the samewavelength as the light emitted by the infrared laser diode. Thiswavelength is filtered out for example by a dielectric filter. Otherwavelengths which for example radiate from the surrounding area onto theeye and are reflected by this or strike the camera directly are therebyfiltered out. The contrast between illuminated and non-illuminated areasor the pupil and its surrounding area can thereby be further increased.

[0041] A further embodiment of the invention is shown in FIG. 1B. Thestructure corresponds to that of the device from FIG. 1A. Here, thefocal point of the beam is directed not onto the pupil but directly ontothe retina. The focal point of the IR source on the retina now generatesfluorescence. A homogenous illumination of the pupil is achieved byspherical waves (indicated by a dotted line), which are now reflected bythe retina. This leads to a preferred regredient illumination. Theretina itself can then function as a light source and not as a lighttrap.

[0042]FIG. 2 illustrates the determination of the eye movement or theeye position using two examples of the eye position. In a first eyeposition 17, represented by a solid line of the iris 7, a first lightbeam 18 is guided via the scanner mirror 12 onto a first imaging point19 of the camera 14. By moving the scanner mirror 11 about its lateralaxis 12 and its vertical axis 13 a complete image of an area of theretina 8 can thus be produced on the camera 14.

[0043] When the eye position changes to a second eye position 20, thesame point of the retina 8 then produces a second light beam 21 whichstrike the camera 14 in a second imaging point 22. When the scannermirror 11 is moved as previously described an image of an area of theretina 8 is likewise produced on the camera. This imaging of the retina8 is shifted vis-a-vis the first eye position 17 on the camera 14. Bycarrying out a suitable mathematical comparison of the digitalizedimages of the camera 14 by means of the evaluation unit 15, the changedangular position of the eye 4 can thus be deduced. The point-focalreflex or scatter point that results on the cornea surface 6 is maskedoff by the image-processing video system of the evaluation unit 15.

List of Reference Numbers

[0044]1 device for tracking eye movement

[0045]2 light source

[0046]3 telescope

[0047]4 eye

[0048]5 focal point

[0049]6 cornea surface

[0050]7 iris

[0051]8 retina

[0052]9 reflected light

[0053]10 pupil

[0054]11 scanner mirror

[0055]12 lateral axis

[0056]13 vertical axis

[0057]14 camera

[0058]15 evaluation unit

[0059]16 op laser

[0060]17 first eye position

[0061]18 first light beam

[0062]19 first imaging point

[0063]20 second eye position

[0064]21 second light beam

[0065]22 second imaging point

[0066]23 filter

1-23. (canceled)
 24. A method for tracking eye movements of an eyehaving a pupil, the method comprising: irradiating an inside of the eyethrough the pupil from outside of the eye so as to excite the inside ofthe eye to illumination; and determining a position of the eye usinglight emitted from the pupil.
 25. The method as recited in claim 24wherein the irradiating is performed using a light source disposedcoaxial with an optical axis of an OP laser.
 26. The method as recitedin claim 25 wherein the light source is an IR laser diode.
 27. Themethod as recited in claim 26 wherein the IR laser diode operates usingpulses.
 28. The method as recited in claim 27 wherein the IR lasersynchronizes the pulses with a laser firing frequency of the OP lasersuch that the IR laser diode illuminates only in laser firing pauses ofthe OP laser.
 29. The method as recited in claim 27 further comprisingabsorbing the light emitted from the pupil using a camera and whereinthe IR laser diode synchronizes the pulses of with an image frequency ofthe camera.
 30. The method as recited in claim 24 wherein theirradiating is performed using a laser diode having a focal point on asurface of a cornea of the eye.
 31. The method as recited in claim 24wherein the irradiating is performed using a laser diode projecting aspot on the retina of the eye.
 32. The method as recited in claim 31wherein the spot is projected to be as broad as possible.
 33. The methodas recited in claim 24 wherein the eye position is determined bydetermining a point of fixation on the cornea.
 34. The method as recitedin claim 33 wherein the point of fixation is a geometric center of anoptical image of the pupil.
 35. The method as recited in claim 33further comprising regularly redetermining the point of fixation. 36.The method as recited in claim 24 further comprising detecting at leastone of a lateral shift of the eye and a rolling movement of the eye byevaluating an image of the pupil.
 37. The method as recited in claim 24wherein further comprising calculating a compensation factor using anaverage eye length and an average anterior chamber depth, thecompensation factor corresponding to a parallax between a front surfaceof a cornea and entrance pupil.
 38. The method as recited in claim 37wherein further comprising transmitting the compensation factor toscanner mirrors of a working laser.
 39. The method as recited in claim37 wherein the compensation factor obeys a linear function.
 40. Themethod as recited in claim 37 wherein the compensation factor obeys anon-linear function.
 41. The method as recited in claim 37 wherein thecalculating of the compensation factor is performed by individualmeasurement of at least one biometric parameter.
 42. The method asrecited in claim 24 wherein the method is performed during refractivelaser surgery.
 43. A device for tracking eye movements of an eye havinga pupil, comprising: a light source configured to irradiate an inside ofthe eye through the pupil so as to excite the inside of the eye toillumination; a camera configured to detect light emitted from thepupil; an evaluation unit operatively connected to the camera andconfigured to determine a position of the eye.
 44. The device as recitedin claim 43, wherein the light source is positioned so that a radiationfrom the light source and a light reflected from the eye run along asame axis.
 45. The device as recited in claim 44, further comprising ascanner mirror positioned on the axis and configured to guide the lightreflected from the eye onto the camera, and wherein the camera is notarranged along the axis.
 46. The device as recited in claim 45, whereinlight from the light source passes through the scanner mirror.
 47. Thedevice as recited in claim 43 wherein the light source emits a lighthaving a wavelength and further comprising a filter disposed in a beampath of the reflected light, the filter configured to admit light havingthe wavelength.
 48. The device as recited in claim 47, wherein thefilter includes a dielectric filter element.
 49. The device as recitedin claim 43, wherein the light source irradiates the inside of the eyeduring refractive laser surgery.